1. Field of the Invention
This invention relates to two-cycle engines and, in particular, this invention relates to exhaust valves for two-cycle engines.
2. Background
Two-cycle engines are widely used for applications such as snow blowers, water craft, all-terrain vehicles, snow mobiles, and the like. The two-cycle engine is generally economically produced because many of the components necessary for four-cycle engines are unnecessary. In contrast to four-cycle engines which use valves, the piston itself blocks and exposes intake and exhaust ports as the piston is reciprocally displaced during operation.
In contrast to four-cycle engines, two-cycle engines have two strokes: 1) intake/compression; and 2) power/exhaust. During the intake/compression stroke in the two-cycle engine, the piston travels upward, thereby generating a low-pressure area in the crank case below the piston and compressing an air/fuel/oil mixture in the cylinder above the piston. Higher atmospheric pressure surrounding the crankcase forces air through the carburetor or throttle valve and, in turn, forces the reed valve open to admit more of the air/fuel/oil mixture to the crankcase When air pressures in the crank case and surrounding atmosphere are approximately equal, the read valve then closes. During the power/exhaust stroke, the piston travels downward, thereby compressing the air or air/fuel/oil mixture in the crankcase. Also during the power/exhaust stroke, the piston movement begins to open transfer ports, thereby forcing the charge from the crankcase, through the transfer ports, and into the cylinder and head chamber. During the foregoing transfer, the exhaust expansion chamber causes a low pressure area to occur at the exhaust port. This low pressure causes spent charge (combusted air/fuel/oil mixture), from the previous stroke, to be drawn out into the exhaust, thereby allowing entry of the new charge.
As the piston travels during the intake/compression stroke the charge pressures are equalized between the cylinder and crankcase. As the piston continues the intake/compression stroke, the transfer ports are closed and the piston begins to compress the fresh charge. At this point, the exhaust port remains open and the piston forces spent charge out through the open exhaust port. As the spent charge is forced through the open exhaust port, a pressure wave is produced. The pressure wave usually impacts a baffle in the exhaust pipe and is deflected back through the exhaust port and toward the cylinder. The effect of the deflected pressure wave is to retain the unspent charge in the cylinder. As the piston continues to travel upwardly, it travels past the exhaust port, thereby trapping the new charge within the cylinder and head chamber for subsequent combustion. Hence, lowering the top (reducing the effective diameter) of the exhaust port causes the new charge to be trapped in the cylinder and head chamber.
In view of the foregoing, one would think that a low exhaust port top (an exhaust port with a relatively small diameter) would always be advantageous. However, a low exhaust port top is actually advantageous only at low rpm when the engine is producing relatively small amounts of exhaust gases. At higher rpm, the exhaust port top must be disposed at a higher position to allow time for the increased combustion pressure to be reduced to less than the atmospheric pressure present in the crankcase. If the cylinder gas pressure is higher than the crankcase gas pressure, exhaust gases will be forced into the crankcase, thereby contaminating the fresh charge and disrupting entry of the new charge into the cylinder. When the exhaust port closes or the exhaust gate is closed by the piston during the compression/intake stroke, the charge is compressed by the piston and at near top dead center, the charge is ignited by the ignition. The piston is then forced down to begin the two stroke cycle anew. Thus, during operation at high rpm, maximum power is attained if the exhaust (combustion products) is efficiently removed from the cylinder. Efficient removal of exhaust gases may be accomplished by comparatively large-diameter exhaust ports. However, a two-stroke engine designed to accommodate expeditious removal of exhaust is often inefficient at low rpm. Inefficiency at low rpm is due in part because the enlargement of the exhaust port diameter diminishes the compression within the cylinder, thereby allowing the uncombusted fuel/oil/air mixture to exit the exhaust port before being combusted. When uncombusted fuel/oil/air mixture is allowed to exit before being combusted, the amount of trapped charge available for combustion is reduced and the efficiency of the engine (as measured by the energy produced per unit fuel) is reduced.
There is then a need for a two-cycle engine which efficiently adjusts the diameter of the exhaust port to a smaller dimension for low rpm and a larger dimension for high rpm.